As this type of heating equipment, there is known a gas recirculating type boiler such as shown in FIG. 6. This gas recirculating type boiler consists of only a convection heat transfer section and has a configuration such that a plurality of tubes 103 for flowing boiler water therethrough are arranged in a furnace 102 and a flame is jetted from a burner 104 provided on one furnace wall directly into the furnace 102 so as to be exhausted from an exhaust opening 109 on the opposed furnace wall. A small combustion space 108 is formed in front of the furnace wall having the burner 104 provided thereon in order to perform combustion and form a flame. When combustion gas weaves between the tubes 103 in the furnace 102, the boiler water in the tube 103 is heated by convection heat transfer. It is to be noted that reference numeral 101 denotes a furnace body; 105, a fuel nozzle; 106, a window box; and 107, a burner throat.
In the prior art gas current type boiler, however, since the combustion gas passes through the furnace 102 in one direction and is directly exhausted, a large difference in temperature of the gas current between the upstream and the downstream of the combustion gas produces an extreme difference in heat flux (heat transfer quantity per heat transfer area) between the tube on the upstream side and that on the downstream side. Therefore, although the average heat flux relative to the heat transfer surface of the entire furnace is reduced and a large heat transfer area is required, an exhaust temperature is high and the thermal efficiency is low. An exhaust temperature must be lowered as possible in order to increase the thermal efficiency. In this case, however, a logarithmic mean temperature difference between the gas current and the tube is decreased as the exhaust temperature becomes lower. Therefore, the more the thermal efficiency is increased by lowering the exhaust temperature, the larger the necessary heat transfer area is required, thereby increasing the size of the furnace.
Further, since the combustion space in the furnace is zero or small if it exists and the flame is immediately cooled down by the water pipe, carbon monoxide (CO) is apt to be generated and the only available fuel is gas fuel, resulting in such a problem as that liquid fuel can not be used.